At temperatures and pressures below its critical point (approximately 374.degree. C.), water is a poor solvent for non-polar materials (including many organic materials) and a good solvent for polar materials (including many inorganic materials). However, at and above the critical temperature of water, many organic compounds become readily soluble in water and many inorganic compounds become insoluble. For example, the solubility of inorganic salts in supercritical water is generally between 1 ppb and 100 ppm above about 450.degree. C., as noted at column 3, lines 39-41 of U.S. Pat. No. 4,338,199 to Modell (hereinafter referred to as "Modell '199").
Supercritical water oxidation ("SCWO") reactors are designed to oxidize organic compounds in water at temperatures and pressures which are above the critical temperature and pressure for water. Under such conditions SCWO reactors are capable of effecting substantially complete oxidation (and hence destruction) of many organic materials, including many toxic organic compounds. The products of such combustion are primarily superheated water, carbon dioxide, inorganic salts and heat. For this reason, SCWO has been proposed as a method of disposing of a wide range of wastes, which contain toxic or noxious organic components. Such wastes include sewage, animal wastes, paper mill wastes and petrochemical wastes. The noxious and toxic compounds suitable for treatment include virtually all oxidizable organic compounds, including dioxins. Supercritical reactors are known and have been described in U.S. Pat. No. 2,944,396 (Barton); U.S. Pat. No. 4,292,953 (Dickinson); U.S. Pat. No. 4,543,190 (Modell et al., hereinafter referred to as "Modell '190"); and others.
Dickinson discloses that a possible limitation to the use of supercritical water oxidation exists in the amount and nature of salts dissolved in the aqueous feed to the reactor. Due to the nature of the supercritical oxidation process, such salts can become concentrated or supersaturated in the reactor. Dickinson further states that with certain types of salts, this concentration effect can result in scaling in the reactor or scaling and/or plugging in downstream heat exchange equipment (column 6, lines 33-47).
The insolubility in water of inorganic compounds at critical conditions has been a major impediment to the development of supercritical water oxidation reactors. Numerous attempts to solve the build-up of scale on the reactor surface have not been successful and have generally required shutting down the reactor and mechanically scrubbing it.
Similarly, Modell '190 states, at column 8, lines 8-34, that in conventional apparatus, inorganics tend to build up on the walls causing hot spots with subsequent destruction of the walls. To overcome this problem, Modell '190 suggests that the inner wall of the reactor be clad with corrosion resistant alloys, such as Hastelloy C, and when high concentrations of inorganic constituents are present, a fluidized bed reactor can be used. However, to do so would greatly increase the cost of the supercritical oxidation reactor.
Several different approaches have been developed to try to overcome this scaling problem. U.S. Pat. No. 4,822,497 (Hong, et al.) discloses a reactor for supercritical water oxidation of organic materials having an upper supercritical zone and a lower subcritical zone. Oxidation of organic materials and inorganic materials, including salts and salt precursors, occurs in the upper zone and salts and other insoluble inorganic precipitates from the oxidation reaction are transferred to the lower subcritical zone when they redissolve and are removed from the reactor as a solution or slurry.
U.S. Pat. No. 5,100,560 (Huang) discloses a supercritical oxidation reactor having an upper supercritical temperature zone and a lower reduced temperature zone. The walls of the reactor are scraped to remove precipitates which deposit on the walls bounding the supercritical temperature zone.
U.S. Pat. No. 5,252,224 to Modell (hereinafter referred to as "Modell '224"), involves a supercritical oxidation process whereby inorganic materials which are insoluble in the oxidation mixture are removed by a combination of mechanical forces and cooling of a downstream portion of the oxidation reactor. This is accomplished by providing the feed stream with a sufficient velocity to prevent the settling out of solid inorganics on the surface of the reactor as the feed stream flows from the inlet to the outlet of the reactor. Such velocity, however, can result in insufficient reaction time of the feed stream to promote complete oxidation thereof within the reactor. Additionally, Modell '224 provides a second zone at the downstream portion of the reactor by providing that portion of the reactor with a cooling jacket which surrounds the reactor tube for purposes of cooling the reactants and forming an effluent mixture which includes gas, liquid, and solid (inorganic) phases. Such reactor designs, however, involve increased costs since a simple tubular reactor must be fitted to account for the additional cooling zone or portion, and further do not compensate for removal of inorganic precipitate and scale which accumulate within the supercritical portion of the reactor.
Modell '224 attempts to further overcome the concerns with the build-up of scale within the reactor through mechanical removal processes which can be accomplished either "on-line" during operation of the reactor, or "off-line", after the reactor is shut down. In the "on-line" process, a wire brush is directed through the interior of the reactor, and is used to scrub the reactor free of scale produced by the deposition of insoluble inorganic material within the reactor. The feed material provides the force for moving the brush within the reactor, causing the brush to dislodge the solid inorganic scale built up on the walls of the reactor and forcing it toward the outlet portion of the reactor. In the "off-line" process, high pressure water is physically directed through the reactor to dislodge the inorganic scale from within the reactor. The high pressure water flushes the dislodged solids from the reactor and into a collection vessel. In further embodiments, a high velocity cleaning spray at supercritical temperatures is used to dislodge the inorganic scale, and finely dispersed abrading solids may be incorporated within the cleaning spray to farther assist in dislodging the scale. While such processes can remove inorganic scale built up within a reactor, they fail to provide a simple effective method which accomplishes removal of the scale in a simple reactor without the need for extraneous equipment and which is capable of producing a simple single phase effluent with solids dissolved therein.
Further, none of these approaches has achieved any commercial success and scaling from the precipitation of inorganic salts remains a major obstacle to the use of SCWO for the treatment of organics in waste streams when the waste stream includes inorganic compounds.